Paper supply device of image forming apparatus and method

ABSTRACT

A paper supply device and method for an image forming apparatus. The paper supply device comprises a driving unit, a first idle gear rotated by the driving unit, a first idle gear rotational shaft which is installed to pass through the first idle gear and rotates with the first idle gear, a paper conveying unit which is installed to be rotated by the first idle gear and conveying paper inward along the paper conveying path, and a multiple-feed preventing unit which is installed to be rotated by the first idle gear and preventing multiple-feed of the paper.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(a) of KoreanPatent Application No. 2003-100468, filed on Dec. 30, 2003, the entirecontents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus and method.More particularly, the present invention relates to a paper supplydevice of an image forming apparatus designed to convey a printingmedium or paper inward along a paper conveying path and a methodthereof.

2. Description of the Related Art

An electrophotographic image forming apparatus such as a laser printer,a facsimile or a copier typically performs an image forming operation bydeveloping an electrostatic latent image formed on a photosensitivemember into a visible image by means of a toner, transferring thevisible image by the toner to paper conveyed from a paper cassette, andfixing the transferred image onto the paper.

For the purpose of this operation, the image forming apparatus isprovided with a paper pick-up unit for sequentially picking up sheets ofpaper loaded in the paper cassette, and a paper supply device forconveying the picked-up paper one sheet at a time toward a paperconveying path formed inside the image forming apparatus.

Therefore, the paper supply device conveys the picked-up sheet of paperinward along the paper conveying path, when the paper pick-up unit picksup a sheet of paper placed on the top of the paper loaded on the papercassette using a pick-up roller and so on.

Occasionally, due to paper-to-paper friction forces and so on, two ormore sheets of paper may be picked up at once and conveyed inward alongthe paper conveying path (hereinafter, a phenomenon that two or moresheets of paper are conveyed is called “multiple-feed”). Accordingly,this multiple-feed brings about various problems, for example a jam,inside the image forming apparatus.

Hence, in recent years, a variety of approaches for preventing themultiple-feed have been proposed.

One example of the approaches is a retard roller method for preventingthe multiple-feed using a torque limiter having a predetermined criticaltorque value, a retard roller connected to the torque limiter and soforth. Here, the torque limiter performs a forward rotation when thepaper feed friction force is greater than the predetermined criticaltorque value, while it performs a reverse rotation when the paper feedfriction force is smaller than the predetermined critical torque value.Further, the retard roller performs either the forward or reverserotation depending on the torque limiter.

To be more specific, the retard roller method prevents the multiple-feedby installing the torque limiter and the retard roller on a lower sideof the paper conveying path along which the paper is conveyed. With thisconfiguration, when either no or one sheet of paper is conveyed alongthe paper conveying path, the paper feed friction force becomes greaterthan the critical torque value of the torque limiter, so that the torquelimiter and the retard roller rotate in the paper feed direction, thusconveying the paper.

In contrast, when two or more sheets of paper are conveyed along thepaper conveying path, the paper feed friction force becomes smaller thanthe critical torque value of the torque limiter, so that the torquelimiter and the retard roller rotate opposite to the paper conveyingdirection. Thereby, it is possible to prevent a multiple-feed.

One example of the paper supply device for the image forming apparatusto which such a retard roller method is employed is disclosed in U.S.Pat. No. 5,571,265 (issued on Nov. 5, 1996) titled “SHEET SUPPLYINGAPPARATUS”.

The disclosed sheet supplying apparatus is provided with a torquelimiter and a retard roller to prevent multiple-feeds of the sheets ofpaper, in which the torque limiter and the retard roller are arranged ona lower side of a path along which the sheets of paper are conveyedsequentially. A retard gear for driving the retard roller is constructedto receive a driving force from a gear, which, in turn, is installed toreceive a driving force from a motor, namely a drive gear coupled to adrive roller, and simultaneously via a plurality of idle gears. Further,the torque limiter, which performs a forward or reverse rotationaccording to paper feed friction force of the paper, is disposed on anintermediate path along which the driving force is transmitted. Thus,for the conventional sheet feeding apparatus, when driving the motorcauses a supply roller and the driver roller to rotate to convey thesheets of paper, the retard gear also receives the driving force throughthe drive gear coupled to the drive roller and the plurality of idlegears engaged with the drive gear, thus rotating in the same directionas that of the drive roller to convey the sheets of paper. However, whentwo or more sheets of paper are conveyed, the rotational directionbecomes different from the previous direction. Specifically, when two ormore sheets of paper are conveyed, the paper feed friction force becomessmaller than a critical torque value of the torque limiter, so that thetorque limiter rotates in a direction opposite to that in which thesheets of paper are conveyed. Therefore, the retard roller also rotatesin a direction opposite to that in which the sheets of paper areconveyed with the torque limiter, thereby preventing multiple-feeding ofthe sheets of paper.

In this manner, the conventional paper feeding apparatus preventsmultiple-feeding of sheets of paper using the retard roller, but itemploys a very complicated gear train in order to drive the retardroller. This causes a problem in that the driving force suffers aconsiderable loss when in use. Moreover, this loss of the driving forceresults in an increase in a load which is applied to the motor.

Further, because the conventional paper feeding apparatus preventsmultiple-feeding of the sheets of paper using the complicated geartrain, it has low operational stability as well as high productioncosts.

SUMMARY OF THE INVENTION

Therefore, an objective of the present invention is to provide a papersupply device for an image forming apparatus, capable not only ofminimizing loss of power, but also of driving a retard roller.

Another objective of the present invention is to provide a paper supplydevice of an image forming apparatus, capable of maximizing operationalstability through a simplified gear train and a method thereof.

In order to accomplish these objectives, there is provided a papersupply device of an image forming apparatus and a method thereof. Theapparatus and method comprise a driving motor, a driving gear coupled tothe driving motor rotating at a predetermined speed; a paper conveyingunit for rotating via the driving gear, installed on any one of upperand lower sides of a paper conveying path, and conveying paper disposedinward along the paper conveying path, and a multiple-feed preventingunit for rotating via the driving gear, installed on the other side ofthe paper conveying path which corresponds to the paper conveying unit,for preventing multiple-feeding of the paper.

Preferably, the driving gear is connected to a first idle gear rotatedby the driving gear, and the paper conveying unit and the multiple-feedpreventing unit are rotated by the first idle gear.

Further, the paper conveying unit, preferably, includes a forward gearwhich is installed to engage with the first idle gear, a forward gearrotational shaft which is installed to pass through the forward gear androtates with the forward gear, and a forward roller which is installedon the forward gear rotational shaft and rotates with the forward gearrotational shaft to convey the paper.

Preferably, the multiple-feed preventing unit includes a retard gearwhich is installed to engage with the first idle gear, a retard gearrotational shaft which is installed to pass through the retard gear androtates with the retard gear, a torque limiter which is installed on theretard gear rotational shaft and rotates with the retard gear rotationalshaft and performs a forward or reverse rotation according to paper feedfriction force of the paper, and a retard roller which is connected tothe torque limiter and rotates with the torque limiter and prevents themultiple-feed of the paper.

Furthermore, it is preferred that the first idle gear is a multi-stagegear and includes a first-stage idle gear provided for engagement withthe forward gear and a second-stage idle gear provided for engagementwith the retard gear, and that the second-stage idle gear has a numberof gear teeth smaller than that of the first-stage idle gear so that theretard gear has a rotational speed slower than that of the forward gear.

Meanwhile, the first idle gear is preferably provided with a first idlegear rotational shaft which allows the first idle gear to be rotatedwith the first idle gear. The first idle gear rotational shaft ispreferably provided with a shaft distance maintaining unit formaintaining at least one of a shaft distance between the first idle gearrotational shaft and the retard gear rotational shaft and a shaftdistance between the first idle gear rotational shaft and the forwardgear rotational shaft within a predetermined range. It is preferred thatthe shaft distance maintaining unit includes at least one coupling platehaving two coupling holes spaced apart from each other by apredetermined distance, the two coupling holes being inserted by thefirst idle gear rotational shaft and a shaft shifting up and down whilemaintaining a predetermined distance from the first idle gear rotationalshaft, respectively.

Further, it is preferred that the coupling plate is further providedwith a pressing means for shifting and pressing one of the retard andforward rollers toward each other so that the retard and forward rollersare brought into contact with each other at a predetermined pressure.The pressing means, preferably, includes an extension spring installedto pull one end of the coupling plate toward the shifted roller, the oneend of the coupling plate being more adjacent to the first idle gearrotational shaft.

Furthermore, it is preferred that the first idle gear rotational shaftis further provided with a plurality of feed rollers which are installedto rotate with the first idle gear rotational shaft and conveys thepaper inward along the paper conveying path.

In addition, preferably, the paper supply device further comprises asecond idle gear between the driving gear and the first idle gear, thesecond idle gear being engaged with both the driving gear and the firstidle gear so as to transmit a rotational force of the driving gear tothe first idle gear.

Hereinafter, a detailed description will be made of a paper supplydevice of an image forming apparatus according to an embodiment of thepresent invention as well as the image forming apparatus to which thepaper supply device is applied.

With regard to the image forming apparatus to which the paper supplydevice of the embodiments of the present invention are applied, itcomprises a main body forming an external appearance and provided with apaper conveying path inside, a paper cassette detachably mounted to themain body and loads a plurality of sheets of paper, a paper pick-up unitmounted to the main body and picking up the sheets of paper loaded inthe cassette one by one from the top sheet of paper, a paper conveyingunit conveying the picked-up paper inward along the paper conveyingpath, an image transferring apparatus transferring a predetermined imageto the conveyed paper, a fixing apparatus semi-permanently fixing thetransferred image, and a central control apparatus controlling the imageforming apparatus as a whole.

Here, two or more sheets of paper are often picked up and conveyed bythe paper pick-up unit. The paper supply device functions not only toconvey the picked-up paper inward along the paper conveying path, butalso to prevent multiple-feed of the picked-up paper.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of embodiments of thepresent invention will become more apparent to those of ordinary skillin the art by describing in detail embodiments thereof with reference tothe accompanying drawings in which:

FIG. 1 is a perspective view illustrating a paper supply device of animage forming apparatus according to an embodiment of the presentinvention;

FIG. 2 is a perspective view illustrating a state where the paper supplydevice of the image forming apparatus shown in FIG. 1 is driven;

FIG. 3 is an enlarged perspective view illustrating a state where afirst idle gear, a forward gear and a retard gear of the paper supplydevice of the image forming apparatus shown in FIG. 1 are assembled; and

FIG. 4 is a perspective view illustrating a paper supply device of animage forming apparatus according to another embodiment of the presentinvention.

Throughout the drawings, it should be noted that the same or similarelements are denoted by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be made in detail with regard to a paper supplydevice 100 of an image forming apparatus according to embodiments of thepresent invention with reference to FIGS. 1 to 4.

As shown in the drawings, the paper supply device 100 of the imageforming apparatus according to embodiments of the present inventioncomprise a driving unit 150 which generates a driving force, a paperconveying unit 120 which is driven by the generated driving force andconveys paper 80 picked-up by a paper pick-up unit (not shown) toward apaper conveying path 70 (see FIG. 2), a multiple-feed preventing unit130 which is driven by the generated driving force and prevents multiplefeeding of the paper, and a driving force transmitting unit whichtransmits the driving force of the driving unit 150 to the paperconveying unit 120 and the multiple feed preventing unit 130.

Specifically, the driving unit 150 includes a driving motor 152 forgenerating a predetermined rotational force, and a driving gear 154coupled to the driving motor 152 and rotating at a predeterminedRevolution per Minute (RPM).

The paper conveying unit 120 is installed on one of upper and lowersides of the paper conveying path 70 and functions to convey the paper80. The paper conveying unit 120 includes a forward gear 122 which isinstalled to engage a first idle gear 170 of the driving forcetransmitting unit to be mentioned below, a forward gear rotational shaft126 which is installed to pass through the forward gear 122 and rotateswith the forward gear 122, and a forward roller 121 which is installedon the forward gear rotational shaft 126 and rotates with the forwardgear rotational shaft 126 to convey the paper 80 inward along the paperconveying path 70. Here, the paper conveying unit 120 is preferablyinstalled on the upper side of the paper conveying path 70.

The multiple feed preventing unit 130 is installed on the other side,corresponding to the paper conveying unit 120, of the upper and lowersides of the paper conveying path 70 and functions to preventmultiple-feeding of the paper 80. The multiple-feed preventing unit 130includes a retard gear 131 which is installed to engage with the firstidle gear 170, a retard gear rotational shaft 135 which is installed topass through the retard gear 131 and rotates with the retard gear 131, atorque limiter 137 which is installed on the retard gear rotationalshaft 135 so as to have a predetermined critical torque value. Thetorque limiter 137 performs a forward rotation when a predeterminedpaper feed friction force is greater than the critical torque value, andperforms a reverse rotation when the predetermined paper feed frictionforce is smaller than the critical torque value, and a retard roller 132which is connected to the torque limiter 137, performs a forward orreverse rotation with the torque limiter, and prevents multiple-feedingof the paper 80. Preferably, the multiple-feed preventing unit 130 isinstalled on the lower side of the paper conveying path 70. Further, theretard roller 132 of the multiple-feed preventing unit 130 is preferablyinstalled on a lower side of the forward roller 121 so as to come intorolling contact with the forward roller 121. Preferably, the torquelimiter 137 performs the forward or reverse rotation depending on thepaper feed friction force of the paper which is conveyed between theforward roller 121 and the retard roller 132. Further, the multiple-feedpreventing unit 130 including the retard roller 132 and the retard gearrotational shaft 135 is preferably installed to shift up and down at apredetermined distance so that the paper 80 can be introduced andconveyed between the retard roller 132 and the forward roller 121 whichare brought into rolling contact with each other.

The driving force transmitting unit includes the first idle gear 170which is engaged with both the forward gear 122 of the paper conveyingunit 120 and the retard gear 131 of the multiple-feed preventing unit130, a second idle gear 160 which transmits the rotational force of thedriving gear 154 to the first idle gear 170, and first and second idlegear rotational shafts 175 and 163 which are installed to pass throughthe first and second idle gears 170 and 160 and rotate with the firstand second idle gears 170 and 160.

Here, the first idle gear 170 is formed of a multi-stage gear, which isengaged with the forward gear 122 and the retard gear 131. To this end,the first idle gear 170 is provided with a first-stage idle gear 171 forengagement with the forward gear 122, and a second-stage idle gear 173for engagement with the retard gear 131 (see FIG. 3). With thisconfiguration, both the forward gear 122 and the retard gear 131 areengaged with the first idle gear 170. The second-stage idle gear 173with which the retard gear 131 is meshed has a smaller number of teeththan that of the first-stage idle gear 171 with which the forward gear122 is meshed. In this manner, the forward and retard gears 122 and 131which are engaged with the first idle gear 170 rotate in proportion tothe number of teeth of the first- and second-stage idle gears 171 and173, and the retard gear 131 meshing with the second-stage idle gear 173rotates at a relatively low speed compared with the forward gear 122meshing with the first-stage idle gear 171.

Further, the first and second idle gear rotational shafts 175 and 163are installed to pass through the first and second idle gears 170 and160 so that the first and second idle gears 170 and 160 can rotaterespectively, and are fixed to a main body 90 of the image formingapparatus. Here, the first and second idle gear rotational shafts 175and 163 are preferably installed on an upper side of the paper conveyingpath 70 similar to the foregoing paper conveying unit 120. Hence, atleast one of the first and second idle gear rotational shafts 175 and163, for example, the first idle gear rotational shaft 175 isadditionally provided with a plurality of feed rollers 140 so as to moresmoothly convey the paper 80 conveyed by the forward roller 121. In thiscase, the paper 80 conveyed by the forward roller 121 is more smoothlyconveyed inward along the paper conveying path 70 by the feed rollers140.

The retard gear 131 of the multiple-feed preventing unit 130 isinstalled to engage with the first idle gear 170. This engagement of theretard gear 131 must continue to be maintained even when themultiple-feed preventing unit 130 shifts up and down at a predetermineddistance, so that the paper 80 can be introduced and conveyed betweenthe retard roller 132 and the forward roller 121.

Therefore, in order to maintain this engagement, the paper supply device100 of the embodiments of the present invention is realized bymaintaining a distance between the shafts of the gears 170 and 131 in apredetermined range. For this purpose, the first idle gear rotationalshaft 175 passing through the first idle gear 170 is provided with ashaft distance maintaining unit 180, which functions to maintain a shaftdistance between the first idle gear rotational shaft 175 and the retardgear rotational shaft 135 in a predetermined range.

The shaft distance maintaining unit 180 is implemented as a couplingplate 185 having two coupling holes 183 spaced apart from each other bya predetermined distance (i.e., a distance which must be maintainedbetween the first idle gear rotational shaft 175 and the retard gearrotational shaft 135), in which one of the two coupling holes isinserted by the first idle gear rotational shaft 175 and the other isinserted by the retard gear rotational shaft 135 as the shaft capable ofshifting up and down while maintaining a predetermined distance from thefirst idle gear rotational shaft 175. In this case, the coupling plate185 is preferably provided with at least two pairs of coupling holes183. In addition, the coupling plate 185 is formed in a “Π” or “U”shape, where both its ends are bent at a predetermined angle. Thesepaired coupling holes 183 are preferably formed on the both ends, whichare bent either downward or upward. Therefore, even when themultiple-feed preventing unit 130 shifts up and down at a predetermineddistance, the shaft distance between the first idle gear rotationalshaft 175 and the retard gear rotational shaft 135 continues to bemaintained by the shaft distance maintaining unit 180 at a predetermineddistance, so that the retard gear 131 and the first idle gear 170continue to be engaged with each other. Meanwhile, the shaft distancemaintaining unit 180 may be equally applied between the first idle gearrotational shaft 175 and the forward gear rotational shaft 126 exceptbetween the first idle gear rotational shaft 175 and the retard gearrotational shaft 135.

Further, after the paper 80 is introduced between the forward roller 121and the retard roller 132, the retard roller 132 installed to come intorolling contact with the forward roller 121 must contact the forwardroller 121 with a predetermined pressure in order to smoothly convey thepaper 80. Therefore, the coupling plate 185 of the shaft distancemaintaining unit 180 is provided with a pressing means, which shifts andpresses one of the retard and forward rollers 132 and 121 toward eachother (herein, one example will be taken from the case of shifting andpressing the retard roller 132 toward the forward roller 121).

The pressing means is preferably implemented as an extension spring 187,which is installed to pull one end of the coupling plate 185 which isinserted by the first idle gear rotational shaft 175 and the retard gearrotational shaft 135 and is closer to the first idle gear rotationalshaft 175. Preferably, one end of the extension spring 187 is hooked onthe one end of the coupling plate 185 which is adjacent to the firstidle gear rotational shaft 175, and the other end of the extensionspring 187 is hooked on the main body 90 located at a lower portion ofthe retard roller 132. Hence, elastic tension of the extension spring187 is applied to the one end of the coupling plate 185, as shown inFIG. 2. As a result, the retard gear rotational shaft 135 inserted intothe coupling plate 185 in parallel shifts upward by a predetermineddistance by a turning movement generated by the elastic tension of theextension spring 187, thus pressing the retard roller 132 toward theforward roller 121.

In the drawings, each of reference numerals 124, 133, 156, 165, 172 and174 represents a gear tooth.

Hereinafter, a detailed description will be provided regarding operationand effects of an image forming apparatus according to an embodiment ofthe present invention as well as the paper supply device 100 of theimage forming apparatus.

When a plurality of sheets of paper 80 are loaded in a paper cassette(not shown) and the paper cassette (not shown) is mounted into the mainbody 90 of the image forming apparatus, the top sheet of paper 80 in thepaper cassette is picked up.

When the top sheet of paper 80 is picked up, the paper supply device 100drives the driving motor 152 so as to convey the picked-up sheet ofpaper inward along the paper conveying path 70 provided in the main body90.

As the driving motor 152 rotates, the driving gear 154 coupled to thedriving motor 152 rotates.

As the driving gear 154 rotates, the second idle gear 160 engaged withthe driving gear 154 rotates, and simultaneously the first idle gear 170engaged with the second idle gear 160 rotates.

Subsequently, the first idle gear 170 rotated by the second idle gear160 rotates the forward and retard gears 122 and 131 in a direction inwhich the paper is conveyed, wherein the forward and retard gears 122and 131 are engaged with the first and second-stage idle gears 171 and173 of the first idle gear 170 respectively.

Thus, the forward roller 121 coupled to the forward gear 122, and thetorque limiter 137 and the retard roller 132 which are coupled to theretard gear 131 rotate to convey the picked-up sheet of paper 80 inwardalong the paper conveying path 70. In this case, the reason the torquelimiter 137 rotates in the direction in which the picked-up sheet ofpaper 80 is conveyed is that the paper 80 conveyed between the tworollers 121 and 131 is one sheet. In other words, the paper feedfriction force of the paper 80 is greater than the critical torque valueof the torque limiter 137.

However, when two or more sheets of the paper 80 are picked up andconveyed in this manner, the paper feed friction force of the conveyedpaper 80 is smaller than the critical torque value of the torque limiter137, so that the torque limiter 137 rotates in a direction opposite tothat of the paper 80. Thus, the retard roller 132 rotates in thedirection opposite to that in which the paper 80 is conveyed with thetorque limiter 137, so that it is possible to prevent multiple-feedingof the paper 80.

Meanwhile, when the paper 80 is conveyed inward along the paperconveying path 70 by the paper supply device 100, an image transferringapparatus (not shown) transfers a predetermined image to the paper 80,and a fixing apparatus (not shown) fixes the transferred image on thepaper 80. Then, the paper 80 is discharged. Thereby, the image formingoperation of the image forming apparatus is completed.

As can be seen from the foregoing, the paper supply device of the imageforming apparatus according to embodiments of the present inventiondrive the retard roller and forward roller using a relatively smallnumber of gears, thus minimizing the energy lost during powertransmission of the gears. Thus, a load applied to the motor is reduced,so that it is possible to considerably save electric power consumed bythe driving of the motor.

Further, the paper supply device of the image forming apparatusaccording to embodiments of the present invention provide numericalminimization and structural simplification of the idle gears to conveythe paper without the multiple-feed, so that it is possible to provideimproved operation stability and minimized production costs.

Furthermore, the paper supply device of the image forming apparatusaccording to embodiments of the present invention maintain the shaftdistance between the first idle gear rotation shaft and the retard gearrotation shaft at a predetermined distance by means of the shaftdistance maintaining unit. Thus, even when the multiple-feed preventingunit with the retard roller shifts up and down at a predetermineddistance, the retard gear can continue to engage with the first idlegear.

In addition, the paper supply device of the image forming apparatusaccording to embodiments of the present invention is provided with thepressing means capable of pressing the retard roller toward the forwardroller, so that the retard roller can continue to come into contact withthe forward roller at a predetermined pressure. Thus, the multiple-feedof the paper conveyed between the retard roller and the forward rollercan be minimized.

Moreover, the paper supply device of the image forming apparatusaccording to embodiments of the present invention is additionallyprovided with the feed roller on the first idle gear shaft, so that thepaper conveyed inward along the paper conveying path by the forwardroller can be more smoothly conveyed by such a feed roller.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to certainembodiments, the described embodiments are simply illustrative of theprinciple of the present invention, and that various other modificationsand changes may be devised. Thus, the scope of protection of the presentinvention should be limited only by the scope and equivalent of thefollowing claims.

1. A paper supply device of an image forming apparatus, comprising: a driving motor; a driving gear coupled to the driving motor for rotating at a predetermined speed; a paper conveying unit for rotating by the driving gear, and conveying paper inward along a paper conveying path; and a multiple-feed preventing unit for rotating by the driving gear, and preventing multiple-feeding of the paper.
 2. The paper supply device according to claim 1, wherein the driving gear is connected to a first idle gear rotated by the driving gear, and the paper conveying unit and the multiple-feed preventing unit are rotated by the first idle gear.
 3. The paper supply device according to claim 2, wherein the paper conveying unit includes a forward gear which is installed to engage with the first idle gear, a forward gear rotational shaft which is installed to pass through the forward gear and rotates with the forward gear, and a forward roller which is installed on the forward gear rotational shaft and rotates with the forward gear rotational shaft to convey the paper.
 4. The paper supply device according to claim 3, wherein the multiple-feed preventing unit includes a retard gear which is installed to engage with the first idle gear, a retard gear rotational shaft which is installed to pass through the retard gear and rotates with the retard gear, a torque limiter which is installed on the retard gear rotational shaft and rotates with the retard gear rotational shaft and performs a forward or reverse rotation according to paper feed friction force of the paper, and a retard roller which is connected to the torque limiter and rotates with the torque limiter and prevents the multiple-feeding of paper.
 5. The paper supply device according to claim 4, wherein the first idle gear is a multi-stage gear and includes a first-stage idle gear provided for engagement with the forward gear and a second-stage idle gear provided for engagement with the retard gear.
 6. The paper supply device according to claim 5, wherein the second-stage idle gear has a number of gear teeth smaller than that of the first-stage idle gear so that the retard gear has a rotational speed slower than that of the forward gear.
 7. The paper supply device according to claim 4, wherein: the first idle gear comprises a first idle gear rotational shaft which allows the first idle gear to be rotated and rotates with the first idle gear; and the first idle gear rotational shaft is provided with a shaft distance maintaining unit for maintaining at least one of a shaft distance between the first idle gear rotational shaft and the retard gear rotational shaft and a shaft distance between the first idle gear rotational shaft and the forward gear rotational shaft within a predetermined range.
 8. The paper supply device according to claim 7, wherein the shaft distance maintaining unit includes at least one coupling plate having two coupling holes spaced apart from each other by a predetermined distance, the two coupling holes being inserted by the first idle gear rotational shaft and a shaft shifting up and down while maintaining a predetermined distance from the first idle gear rotational shaft, respectively.
 9. The paper supply device according to claim 8, wherein the coupling plate is further provided with a pressing means for shifting and pressing one of the retard and forward rollers toward the other so that the retard and forward rollers are brought into contact with each other at a predetermined pressure.
 10. The paper supply device according to claim 9, wherein the pressing means includes an extension spring installed to pull one end of the coupling plate toward the shifted roller, the one end of the coupling plate being more adjacent to the first idle gear rotational shaft.
 11. The paper supply device according to claim 7, wherein the first idle gear rotational shaft is further provided with a plurality of feed rollers which are installed to rotate with the first idle gear rotational shaft and conveys the paper inward along the paper conveying path.
 12. The paper supply device according to claim 2, further comprising a second idle gear between the driving gear and the first idle gear, the second idle gear being engaged with both the driving gear and the first idle gear so as to transmit a rotational force of the driving gear to the first idle gear.
 13. The paper supply device according to claim 1, wherein the paper conveying unit is installed on any one of upper and lower sides of a paper conveying path.
 14. The paper supply device according to claim 1, wherein the multiple-feed preventing unit is installed on the other side of the paper conveying path which corresponds to the paper conveying unit.
 15. A method for supplying paper for an image forming apparatus, comprising: providing a driving motor; coupling a driving gear to the driving motor for rotating at a predetermined speed; rotating a paper conveying unit via the driving gear, and conveying paper inward along a paper conveying path; and rotating a multiple-feed preventing unit via the driving gear, and preventing multiple-feeding of the paper.
 16. The method according to claim 15, further comprising: connecting the driving gear to a first idle gear rotated by the driving gear; and rotating the paper conveying unit and the multiple-feed preventing unit via the first idle gear.
 17. The method according to claim 16, wherein the paper conveying unit comprises a forward gear.
 18. The method according to claim 17, further comprising: engaging the forward gear with the first idle gear.
 19. The method according to claim 18, further comprising: providing a forward gear rotational shaft.
 20. The method according to claim 19, wherein the forward gear rotational shaft passes through the forward gear.
 21. The method according to claim 20, further comprising: rotating the forward gear rotational shaft with the forward gear.
 22. The method according to claim 19, wherein the paper conveying unit comprises a forward roller which is installed on the forward gear rotational shaft.
 23. The method according to claim 22, further comprising: rotating the forward roller via the forward gear rotational shaft in order to convey the paper. 